Oh, I assumed OP was flying LAX-AMS-VIE//VIE-AMS-LAX and planning on skipping the AMS-LAX segment.

Regardless, no-showing for ANY segment (second, last, or otherwise) will result in cancellation of your itinerary, leaving you stuck in AMS. How long are you planning on staying there? You can change to a long layover or to a stopover.